In conventional fiber blending and yarn manufacturing procedures, the fibers are generally submitted to tensioning forces by which they are broken up to a certain extent. As long as the fibers to be blended have approximately the same stress-strain characteristics no substantial problems arise during blending or in obtaining the desired and controllable break-up and homogeneity of the blend.
However, on blending fibers with substantially different elasticity modulus, during which process all the fibers are normally subjected to approximately the same tension forces or tensile stresses, the fibers with a low elasticity modulus will tend to elongate substantially more than the fibers with a high elasticity modulus.
At the end of each blending step, when the tension on the composite fiber bundle is removed, the fibers with a low modulus will consequently tend to retract more than the fibers with a high modulus, and as both kinds of fibers are in frictional engagement with each other, the fibers with high modulus will be subjected to certain axial compression type forces. Accordingly, these fibers will be forced to curl more or less about the retracting fibers, which remain relatively straight. This curling effect must substantially be avoided, as it tends to decrease the degree of homogeneity of the blend in an uncontrollable way. Moreover, we must take into account, that this curling effect will be enhanced proportionally to the volume percentage of the low modulus fibers present in the blend. Consequently, control of the homogeneity of the resulting blend can become more critical as the percentage of the high modulus fibers in the blend is reduced. When the high modulus fibers additionally have a greater stiffness or a greater bending modulus than the low modulus fibers, which is often the case, it will be difficult and even quite impossible to pick or unravel the entangled mass or clustered slubs of high modulus fibers within the bundle, during further blending steps. In fact, contrary to what is desired, further blending on pin drafters, for example, can enhance the curling, entangling and clustering of the high modulus fibers to form knots within the sliver or fiber bundle, rather than unraveling the slubs.
Further, when choosing the diameters of the low and high modulus fibers so that they have about the same breaking load, there will of course be a tendency for the high modulus fibers to break up much more in short pieces than the neighboring low modulus fibers. Attempts have already been made to control the break-up of the high modulus fibers, but most such attempts have failed more or less. In U.S. Pat. No. 3,670,485 there is a proposal to deliver metal filament bundles (high modulus fibers) to partially carded textile fiber webs and further carding this combination to break the metal filaments into short fibers and blend them substantially uniformly with the textile fibers. However, the break-up of the high modulus fibers is generally too drastic and a substantial amount of these fibers are torn almost to dust and lost from the card web before this web is condensed and wound up.
In another instance, high modulus metal fiber slivers or tows were sandwiched between textile slivers at the input of a draw frame with the aim being to cushion the high modulus fibers between the adjacent low modulus fibers and so to counteract severe crushing and uncontrollable rupture of the high modulus fibers. This is suggested in "Modern Textiles Magazine"--June 1967, page 54, top of third column. However, this method requres that multiple consecutive drawing and blending steps are required either to come to a desirable low blending percentage of high modulus fibers or to effect a sufficient breakdown of the high modulus fibers or both. This is due to the fact that the bundle of high modulus fibers to be introduced should be of sufficient size to be handled and moved without rupture in view of the average fiber length in it. Hence the actual blending method on the draw frame, although generally useful in obtaining a desired blend, is quite unproductive, certainly in those instances where a very low percentage of high modulus fibers in the blend is desirable.